Body fluids are mainly withdrawn for the purpose of subsequent analysis, in order to permit diagnosis of diseases or to monitor a patient's metabolism. This kind of withdrawal is carried out on diabetics in particular, in order to determine the blood sugar concentration.
In the prior art, blood sampling systems have for some time been known with which the patient or hospital staff can withdraw fluid in a simple manner. An example of an appliance suitable for this purpose is the commercially available Softclix lancing device, whose operation is described in U.S. Pat. No. Re. 35,803. This appliance provides a possibility of setting the depth of insertion of a lancet into the tissue. Thus, the patient is able to select the minimum depth of insertion with which he obtains just the right amount of blood for subsequent analysis, and he can thus minimize the pain caused by puncturing the skin. After the patient has created an opening in the skin by puncturing it, then, particularly in the case of shallow insertion depths, he has to rub or apply pressure to the finger in order to extract sufficient blood from the puncture wound.
A further system for withdrawing body fluid is known from U.S. Pat. No. 5,857,983. This system comprises a hollow puncturing element for creating an incision at the withdrawal site, and a collection tube which communicates with the puncturing element. A so-called stimulator incites flow of body fluid from the puncture by compressing a ring of body tissue around the puncture. During this process, the puncturing element is moved within the incision by means of a movement mechanism in order to keep the incision open during withdrawal of the body fluid. A suction mechanism sucks the body fluid through the puncturing element into the collection tube. In this appliance too, it is possible to set the depth of insertion of the puncturing element.
U.S. Pat. No. 5,951,492 relates to a further system for withdrawing body fluid. It comprises a mechanism for creating a puncture in the skin of the user, and a sample-withdrawing element in order to convey the body fluid through the puncture and via a capillary element onto a test strip, which receives the body fluid from the capillary element. This system also comprises a sample-identifying mechanism which detects a droplet of body fluid on the surface of the user's skin.
U.S. Pat. No. 6,319,210 relates to another system for withdrawing body fluid, in particular blood or interstitial fluid. This document describes a puncturing mechanism which is also intended to be suitable for withdrawal sites other than the finger pad. Here once again, a stimulator is used to incite the body fluid to flow from an incision, the stimulator either being heated or vibrated.
PCT Patent Publication No. WO 01/89383 discloses a system for withdrawing body fluid from an area of a body part, in particular the finger pad, with a compression unit onto which the body part is pressed in a primary direction and which converts the pressure partially into a movement in a secondary direction, partially transverse to the primary direction, so that the internal pressure in an area of the body part is increased. This system further comprises a puncturing device, in particular a lancet or cannula, for creating an opening in the body in the area of increased internal pressure, the compression unit having a press-on area made of a deformable material.
However, the systems from the prior art are not able to detect the withdrawal conditions, in particular the location of the withdrawal and the state of the skin at the withdrawal site, nor are they able to adjust automatically to these withdrawal conditions in order to ensure that body fluid can be withdrawn as painlessly as possible and in as short a time as possible, with the fewest possible maneuvers by the user.
Different withdrawal sites (for example finger pad or forearm) require different parameter sets for withdrawal of the body fluid and also for its analysis. The volume of blood attainable from the forearm is very much smaller. Furthermore, the puncture needed is much deeper compared to when collecting a sample from the finger. At the same time, there is less need for precise control of the puncture depth, because the overall sensitivity to pain is lower and is influenced less by the puncture depth. If the parameter set for withdrawal of blood from the forearm was applied for withdrawing blood from the finger pad, this would involve an extremely deep and thus very painful incision. Further disadvantages are of a hygienic nature, because incorrect use of the forearm puncture parameters leads to far too great a sample volume, as a result of which the system may be soiled.
Moreover, the parameter sets ought to be adapted to the state of the skin at the withdrawal site, for example to the skin temperature. Generally, the warmer the skin, the better its blood circulation, and the easier it is to collect blood from it.
For substantially pain-free withdrawal of body fluid, it is also important that this is done only when the withdrawal site is lying correctly on the withdrawal system, that is to say with sufficient pressure and over a sufficiently large surface area.
It is therefore an object of the present invention to make available a system and a method for withdrawing body fluid, which system and method avoid the aforementioned disadvantages of the prior art and permit substantially painless withdrawal of body fluid in a short space of time and with the fewest possible maneuvers by the user. They are also intended to permit withdrawal of body fluid at different withdrawal sites (alternate site testing) and with different states of the withdrawal site, the system being intended to automatically adjust to the withdrawal conditions.